Silicon nitride and SiAlON ceramics are engineering ceramic materials which are characterised by an excellent combination of mechanical properties of stiffness, strength, hardness and toughness which can, in theory, be retained to very high (>1000° C.) temperatures.
The SiAlONs are based on compositions containing the elements Si, Al, O, N, hence the acronym. The most successful commercial SiAlON (beta SiAlON) has the beta-Si3N4 crystal structure, but with some of the silicon atoms replaced by aluminium atoms and the same number of nitrogen atoms replaced by oxygen atoms to form Si6-zAlzOzN8-z where 0<z<4.2. The other common SiAlON phase is alpha SiAlON, which has the general composition MxSi12-m−nAlm+nOnN16-n, where m represents the number of Si—N bonds replaced by Al—N per unit cell, n represents the number of Si—N bonds replaced by Al—O per unit cell, 0<x<2, and M is one of the cations including Li, Mg, Ca, Y and rare earths (excluding La, Ce).
Beta SiAlON is a strong engineering ceramic with good oxidation and creep resistance up to 1300° C. Alpha SiAlON has excellent hardness, but slightly worse strength, toughness and oxidation resistance than beta SiAlON. By selecting a particular phase, it is possible to define quite precisely an optimum combination of mechanical properties. Combinations of alpha-beta SiAlONs are in thermodynamic equilibrium and so optimised composite materials can be produced in this way.
SiAlONs are usually formed by mixing Si3N4, Al2O3, ALN powders with one or more metal oxides (often including Y2O3), compacting the powder to the desired shape, and then firing the component at 1750° C. for a few hours. The function of the metal oxide is to react with the silica, always present on the surface of each silicon nitride particle, to form a liquid phase, which assists densification. After sintering, the liquid phase, which also contains nitrogen, cools to form an amorphous phase between the SiAlON grains. In subsequent use of these materials, the amorphous phase starts to soften at temperatures slightly above its glass transition temperature (Tg) and the mechanical properties deteriorate rapidly. Even with the most refractory oxide additives, (Tg) is barely in excess of 1000° C.
In an attempt to provide a ceramic SiAlON composition, which is usable in high temperature applications, prior art methods and compositions have taught the combination of alpha SiAlON, beta SiAlON and intergranular phases.
U.S. Pat. No. 4,563,433 and U.S. Pat. No. 4,711,644 disclose a ceramic containing alpha SiAlON, beta SiAlON and an intergranular phase. This alpha SiAlON phase is formed by using yttrium and/or other rare earth elements.
U.S. Pat. No. 5,200,374 discloses a ceramic containing alpha SiAlON, beta SiAlON and intergranular phase. This alpha SiAlON phase is formed by using rare earth elements selected from the group consisting of Ho, Er, Tm, Yb and Lu.
U.S. Pat. No. 5,227,346 discloses a ceramic containing alpha SiAlON, beta SiAlON and intergranular phase. This SiAlON material is formed by using a compound selected from the group consisting of oxides and nitrides of Sr, at least one of Ca, Mg, Li or Na and at least one of yttrium or rare earth elements.
The mentioned prior arts use yttrium and/or rare earth cations except U.S. Pat. No. 5,227,346. Although using yttrium and/or rare earth cations gives required multi-phase SiAlON ceramic materials, microstructure of these ceramics and to greater extent mechanical and/or thermal properties may not be the desired ones. This can be explained by transformation of alpha SiAlON to beta SiAlON in use where there is a depletion of alpha SiAlON phase.
Primary objective of the present art is to produce a multi-phase SiAlON material with improved properties and stable microstructure at high temperatures. It is also a further objective to design a composition in which the amount of glassy phase can be minimised by using suitable combination of cations.